This invention relates to a method of making self-supporting thin, curved rare earth-transition metal (RE-TM) magnets wherein the magnetic domains are substantially radially magnetically aligned. More particularly, this invention relates to a novel method of sintering and densifying green RE-TM powder compacts between specially suited dies to form magnets which are particularly useful as pole pieces for D.C. motors.
Rare earth-transition metal permanent magnets are well known for having magnetic energy products markedly higher than those of conventional permanent magnets. These properties make RE-TM magnets particularly useful as pole pieces for high torque D.C. motor applications. The size and weight of motors equipped with RE-TM magnets can be substantially reduced over conventional D.C. motors which require heavy copper windings or bulky iron or ferrite magnets. Motors with RE-TM pole magnets would be particularly useful in automotive applications such as window-lift motors, windshield wiper motors, starter motors, etc. For a motor with a cylindrical motor casing, the RE-TM magnet pole pieces should form arc segments of a thin walled cylinder sized to fit inside the casing with the magnetic domains of the magnets substantially radially aligned with respect to the axis of the cylinder.
A major obstacle to the widespread use of RE-TM magnets in D.C. motors has been the need for a commercial process for making thin, curved, self-supporting magnets. One practice has been to grind flat magnets having magnetic domains aligned in a perpendicular direction with respect to the flat surface into a thin, curved shape. Such grinding is time consuming and wasteful of relatively expensive rare earth-transition metal materials. Moreover, the direction of magnetic alignment of the resulting magnets is not optimal for the shape of the device in which it is to serve.
Another approach has been to deform a flat, sintered slab magnet into a curved shape. See, e.g., U.S. Pat. No. 3,864,808. The flat predensified magnets are heated to a temperature below the sintering temperature of the magnet but at which plastic deformation takes place under pressure exerted by a forming die resting on top of the magnet. Two major problems with this process are that the magnets must be deformed slowly to prevent them from breaking or distorting and it is only effective for shaping very thin, small magnets.